Variable-frequency tuned-cavity coupler



Nov. 19, 1968 D. J. LARGE ETAL VARIABLE-FREQUENCY TUNED CAVITY COUPLER Filed April 22, 1966 5 Sheets-Sheet 1 INVENTORS DAV/0 J. LARGE RICHARD H. SWART LE Y ATTORNEY Nov. 19, 1968 VAR IABLE-FREQUEINCY TUNED CAVITY COUPLER Filed April 22, 1966 3 Sheets-Sheet 2 50 70 T fl l 62 i L 88 I I 74 56 I f 42 78 a4 56 64 Q 7a 40 76 a /0s as w 22 i 1NVENTOR5 DAV/D J. LARGE RICHARD H. SWARTLEY ATTORNEY I Nov. 19, 1968 D. J. LARGE ETAL VARIABLE-FREQUENCY TUNED-CAVITY COUPLER Filed April 22, 1966 s Sheets-Sheet s wa l I \\\\\\7 \2 82 74 8 4 36 F Tg. 5'

F 2 7 5 INVENTORS 04 W0 J. LARGE BY RICHARD H. S WART L E' Y ATTORNEY United States Patent 3,412,341 VARIABLE-FREQUENCY TUNED-CAVITY COUPLER David J. Large, San Jose, Calif., and Richard H. Swartley, Straiford, Pa., assignors to Varian Associates, Palo Alto,

Calif., a corporation of California Filed Apr. 22, 1966, Ser. No. 544,566 Claims. (Cl. 33056) ABSTRACT OF THE DISCLOSURE A variable-frequency tuned-cavity coupler for microwave tubes is disclosed in which a stripline is used as a major portion of the tuned cavity. The stripline is disposed in an arcuate manner to enable tuning to be accomplished by rotary movement of shorting means along the arcuate stripline, thus producing a compact tunable resonant cavity structure.

This invention relates to tuned-cavity microwave couplers, and more particularly to a stripline coupler capable of coupling the input of an amplifier tube to a coaxial cable carrying the input signal, throughout a wide range of frequencies.

In various kinds of microwave apparatus such as test equipment, it is often desirable to amplify a signal from a microwave oscillator tunable over a wide range of frequencies, to a power level at which it can be of practical use. This amplification is normally achieved by the use of a triode tube of the grounded grid type. Such a tube has a coaxial configuration at microwave frequencies, with the grid connection being the outer conductor of the coaxial housing, the plate connection being the inner conductor at one end, and the cathode connection being the inner conductor at the other end. Filament voltage is normally supplied to such a tube by bringing a heater in through the hollow cathode.

As long as the tube is to be operated at a single frequency, the electrode cavities can have a fixed length, and coupling an input signal to the cathode cavity is simple. However, when the tube is to be operated through a wide range of frequencies, proper coupling of the signal input cable to the cathode cavity becomes a serious problem, and it is this problem that the present invention is designed to solve.

Inherently, a microwave triode tube has a relatively low-Q input. Consequently, points at which the impedance of a cathode resonator cavity matches or exceeds the 50 ohm rating of the usual coaxial cable,-as is necessary for proper coupling, are found only reasonably near the peaks of the standing wave pattern set up when the cavity is tuned to the frequency to be amplified. When the coupler is operated in a quarter-wave mode, the problem is not senious because a fixed probe position can usually be found in the coupler structure at which the input impedance is in excess of 50 ohms through a substantial range of frequencies. However, as the tube is tuned through higher and wider ranges of frequency, the first node disappears into the tube, and the resonator has to be operated in agmultiple quarter-wave mode. When this is done, the displacement of the outer nodes with frequency becomes so great that a fixed probe location is no longer reasonably feasible.

Various types of movable probes have been devised in the past, but they all have the disadvantage of requiring the input connection to be movable. This is undesirable from several points of view, because the movement of a coaxial input cable not only creates electrical problems, but also results in metal fatigue which shortens the life See of the equipment. In addition, all the prior art devices are of excessive length, because a straight resonator with a movable short and movable tap must inherently be at least a quarter wavelength long at the lowest frequency, and it usually requires additional room to accommodate,

beyond its end, operating mechanism of at least equal length. For example, a prior art amplifier tunable through a range of to 1000 megacycles would, asa practical matter, require some seven feet of space and would be complex and expensive to manufacture.

The present invention solves the problem by using, as the major portion of the resonant cavity, a stripline disposed along a circular path. This circular arrangement of the stripline portion of the resonant cavity not only permits the packing of three feet of cavity into a one-foot space, but it also permits the use of rotating taps and shorts which in turn allow the input connection to remain stationary while the tap and short are moved.

It is, therefore, the object of this invention to provide a tuned-cavity coupler for microwave tube cathode circuits which permits a wide range of tuning within a relatively small space.

It is a further object of this invention to provide a coupler of the type described which has a movable tap, yet in which the input connection remains fixed during movement of the tap.

It is another object of this invention to provide a conpler of the type described which is simple and inexpensive to construct, yet which is highly reliable and efficient from an electrical point of view.

These and other objects of the invention will become apparent from the following specification, taken in connection with the accompanying drawings in which:

FIG. 1 is a perspective view of the coupler of this in vention as mounted into the cathode circuit of a microwave amplifier tube;

FIG. 2 is a schematic representation illustrating the electrical functioning of the invention;

FIG. 3 is a vertical axial section of the device of this invention;

FIG. 4 is a vertical section along line 44 of FIG. 3;

FIG. 5 is a fragmentary enlarged section along line 55 of FIG. 4;

FIG. 6 is a fragmentary enlarged section along line 66 of FIG. 4; and

FIG. 7 is a plan view of the underside of the shorting arm.

Basically, the device of this invention operates by matching the cathode end of the amplifier tubes coaxial housing to an arcuate stripline through a stationary coaxial-to-stripline adapter of well known construction to form a resonant cavity. The stripline preferably forms an arc of a circle but may have a spiral or other areuate configuration as the exigencies of a particular use may require. A short and a tap positioned on concentric shafts can be turned by a pair of concentric knobs to any position on the stripline to vary the resonant frequency and coupling impedance. The tap is connected through a coaxial cable to a rotary joint of known construction, where it is mated to a fixed connector to which the input signal carrying coaxial cable can be attached. Provisions are made to supply filament current to the tube through a direct current lead positioned inside the hollow center conductor of the stripline and shielded at its exit points against microwave-frequency pick-up from the microwave-frequency portions of the circuit.

Referring now to the drawings, the tuned-cavity coupler of this invention is generally shown at 10. The coupler 10 can be adjusted to the proper impedance by a coupling control 12 and a tuning control 14. In use, the coupler 10 is mounted directly onto the cathode end of an amplifier tube whose grounded grid conductor is shown at 16, whose cathode conductor is shown at 18, and whose filament supply wire is shown at 20. The input signal is brought into the coupler by means of a coaxial cable 22, and DC filament voltage is supplied through the wires 24.

FIG. 2 explains the electrical functioning of the invention. In FIG. 2, the bracket 26 denotes the tube, whereas the bracket denotes the coupler. The tube 26 has a plate 28, a grounded grid 30, a cathode 32, and a filament 34. One end of the filament 34 is connected to the cathode conductor 18, and the other end of the filament 34 is connected to the wire 20. At their lower ends, the cathode conductor 18 and the grid conductor 16 are connected without substantial discontinuity, by means of a coaxial-to-stripline adapter 36, to a stripline 39 represented in FIG. 2 by the ground plane 38 and the hollow center conductor 40. The generic form center conductor is used herein to designate element 40 even though the stripline 39 is a two-layer line. It will be understood that the grid conductor 16 is connected to the ground plane 38, and that the cathode conductor 18 is connected to the center conductor 40 of the stripline 39, as symbolically shown by the dotted lines inside the adapter 36 in FIG. 2.

Tuning of the cathode cavity of the tube 26 is accomplished by moving a movable short 42 along the stripline 39 so as to set up a standing wave having an odd number of quarter wavelengths in the distance between the short 42 and the gap containing the electrodes of the tube 26. Such a standing wave is illustrated at 44 in FIG. 2. The short 42 is equipped with a capacitor 46 to prevent grounding of the DC supply 24.

With the short 42 set at the distance S from the gap to produce resonance at the desired frequency, the tap 48 is next set at the distance T from the gap at which the signal input 22 sees an optimum impedance for maximum coupling. In the example described herein, the signal input line 22 is coupled to the stripline 39 by direct electrical contact for simplicity of design and ease of manufacture, but it should be understood that the coupling between the signal input line 22 and the stripline 39 may equally well be capacitive or inductive if the physical configurtion of the stripline so permits. The outer conductor of the signal input cable 22 is connected to the ground plane 38 of the stripline 39 by a wiper 50, whereas the inner conductor of the signal input cable 22 is connected through a capacitor 52 to the wiper 54 which runs on the center conductor 40 of the stripline 39. The purpose of the capacitor 52 is, of course, to prevent DC feedback into the signal input circuit. The two leads of the DC line 24 are grounded to the ground plane 38 through capacitors 56, 58 to prevent stray microwave-frequency signals from coupling into the DC line 24.

FIGS. 3 through 6 illustrate the physical arrangement by which the electrical objectives of FIG. 2 are carried out in accordance with the invention. In a preferred embodiment of the invention, the stripline 39 is housed within a disc-shaped housing 59 which also forms the ground plane 38 of the stripline 39. The housing 59 is closed off by a cover 60 (FIG. 3) which supports the roller bearing 62 for the outer one of a pair of concentric control knob shafts 64, 66. The control knob shaft 66 is supported on the housing 59 by a roller bearing 68, whereas the shaft 64 is supported on shaft 66 by a roller hearing 70.

Mounted around the periphery of the bottom of the disc-shaped housing 59 is a strip 72 of dielectric material which provides the dielectric separation between the ground plane 38 and the ring-shaped hollow center conductor 40 of the circular stripline 39.

The clockwise end (in FIG. 4) of the stripline 39 terminates into an adapter 36 which matches, in accordance with conventional waveguide design criteria, the stripline 39 to a coaxial tube socket 74. The tube socket 74 is provided with terminal clips 16a, 18a, and a which snap over the conductors 16, 18 and 20, respectively, of the tube 26. The wire 20 is protected against stray microwave pick-up between the socket 20a and the interior of center conductor 40 by a shield 76 which encloses it on all sides.

The counter-clockwise end of the stripline 39 terminates into a shield 78 which prevents microwave energy from coupling into the interior of the coupled housing. The DC leads 20 (from the interior of the hollow conductor 40) and 78 (from the conductor 40 itself) are brought out through the bottom of housing 59 by capactive fittings which constitute the capacitors 56, 58 of FIG. 2.

The effective electrical length of the stripline 39, and hence the resonant frequency, is determined by augular position of a shorting arm 42, which is mounted on the outer control knob shaft 64 for movement along the stripline 39, and is shown in more detail in FIG. 6. The shorting arm 42 is provided with two pairs of wipers: the ground plane wipers 82, 84, and the center conductor wipers 86, 88. The ground plane wiper 82 is connected to the center conductor wiper 86 through a dielectric 90, and likewise the ground plane wiper 84 is connected to the center conductor wiper 88 through a dielectric 92. The dielectrics 90, 92 constitute the capacitor 46 of FIG. 2. It will be understood that this arrangement provides a short at microwave frequencies while blocking the grounding of the direct current filament potential of center conductor 40.

Referring now back to FIG. 3, the signal input line 22 which may, for example, come from an oscillator (not shown) is connected to a fixed fitting 94 mounted on the rotary joint housing 96. The housing 96 contains a conventional rotary joint 98 which matches a stationary fitting 99 fixed with respect to housing 96 to a rotatable fitting 102 capable of turning with the coaxial cable when that cable is rotated by rotation of the inner knob shaft 66. Within the fitting 102, the inner conductor of coaxial cable 100 is interrupted by a DC blocking capacitor (not shown in FIG. 3) which is the capacitor 52 in the schematic illustration of FIG. 2.

The coaxial cable 100 enters the housing 59 through an opening 104 in the center thereof. It then passes through an opening 106 in the wall of inner control knob shaft 66, outside of which it is attached to the tap arm 48. At the outer end of the tap arm 48, the center conductor of coaxial cable 100 is connected to wipers 54 which ride against the side of conductor 40, whereas the outer conductor of coaxial cable 100 is connected to the ground plane wipers 50 which ride on the ground plane 38. The tap arm 48 is fastened to the inner control knob shaft 66 and rotates with it.

Preferably, the arms 42 and 48 are not exactly radial, but rather disposed at a slight angle to the radius as best shown in FIG. 4. The reason for this arrangement is to allow the short and tap to be moved into close proximity to one another to obtain greater flexibility in coupling.

It will be seen that by mounting appropriate control knobs 12 and 14 on shafts 66, 64 respectively, the tuning and the coupling of the tube 26 can be readily adjusted by turning the knobs 14, 12 to move the shorting arm 42 and the tap arm 48, respectively, without the necessity of moving the input cable fitting 94. Inasmuch as the circumference of a cable is 1r times its diameter, it will be readily seen that a circular stripline of the type described can provide an electrical length of over three times its diameter. In addition, the fact that both the shorting arm 42 and the tap arm 48 are moved by rotational motion from the center of the stripline circle rather than by a motion longitudinally of it, makes it unnecessary to provide any operating room beyond the end of the stripline.

It will be seen that the above-described invention can be carried out in many different ways of which the embodiment shown is merely illustrative. Consequently, we do not desire to be limited by the structure shown as an embodiment herein, but only by the scope of the following claims.

We claim:

1. A variable-frequency tuned-cavity coupler for microwave tubes, comprising:

(a) means for receiving conductors from electrodes of a microwave tube;

(b) stripline means disposed in an arcuate path and electrically matched to said conductor-receiving means;

(c) movable shorting means for shorting said stripline at microwave frequencies; and

(d) movable tap means connected to a microwave signal source and movable along said stripline to variably couple said source to said tube.

2. The coupler of claim 1, in which said stripline is disposed in an arcuate path consisting of at least a part of at least one convolution about an axis, and said shorting means and tap means are mounted for rotational movement about said axis.

3. The coupler of claim 2, in which said path is an arc of a circle.

4. The coupler of claim 2, further comprising:

(e) generally rigid coaxial line means connected to said tap means for movement therewith and positioned so as to have a portion generally coaxial with said axis;

(f) fixed input connector means; and

(g) rotary connector means electrically matching said portion to said fixed input connector means regardless of the position of said coaxial line means.

5. A variable-frequency tuned-cavity coupler for microwave tubes, comprising:

(a) a coaxial tube socket;

(b) a stripline generally disposed in an arc of a circle;

(c) adapter means matching said tube socket to said stripline;

(d) a pair of control means coaxially rotatable about an axis passing through the center of said circle;

(e) a shorting arm supported by one of said control means and rotatable therewith, said shorting arm carrying means riding on said stripline to short the same at microwave frequencies at selectable positions to vary its effective electrical length; and

(f) a tap arm supported by the other of said control means and rotatable therewith, said tap arm carrying a coaxial line and means for coupling said line to said stripline at selectable positions to vary the effective impedance seen by said line.

6. The coupler of claim 5, further comprising:

(g) a fixed connector; and

(h) rotary connector means arranged to match said coaxial line to said fixed connector regardless of the rotational position of said coaxial line.

7. The coupler of claim 6, further comprising capacitive means between said coaxial line andsaid fixed connector to block passage of direct current therebetween.

8. The coupler of claim 5, in which the center conductor of said stripline is hollow and serves as a passageway for a tube filament lead. i

9. The coupler of claim 8, in which said adapter means include shield means connected to the center conductors of said stripline and tube socket and completely enclosing said filament lead between said socket and said stripline.

10. The coupler of claim 8, in which said filament lead is shorted to the ground plane of said stripline at microwave frequencies adjacent the end of said stripline remote from said adapter means.

11. The coupler of claim 5, further comprising shield means enclosing the end of said stripline remote from said adapter means.

12. The coupler of claim 5, in which said coupling means of said tap arm are of the direct contact type.

13. The coupler of claim 5, in which said shorting means on said shorting arm include capacitive means arranged to block direct current flow through said shorting means.

14. The coupler of claim 5, in which said stripline, adapter means and arms are enclosed in a shielding housing which constitutes the ground plane of said stripline.

15. The coupler of claim 5, in which said arms are disposed at an angle to the radius of said circle to permit close physical approach of their outer ends.

References Cited UNITED STATES PATENTS 10/1964 Weaver 330-56 6/1967 Clark 33056 

